Ophthalmic lens



pri 13, 1937. E. D.- Tl-LLYER OPHTHALMIC LENS Filed April 14, 1954 m' R en Patented Apr. 13, 1937 OPHTHALMIC LENS Edgar D. Tillyer, Southbridge, Mass., assigner to American Optical Company, Southbridge,

vMass., a voluntary association setts of Massachu- Applicatin April 14, 1934, serial No. 720,594

'8 Claims.

This invention relates to improvements in opththalmic lenses, and relates particularly to ophthalmic lenses used for the equalization of the mental impressions of size in the two eyes,

said impressions being also referred to in the art as ocular images.

One of the principal objects of the invention is to provide means of separating in a lens or lens system the size and the focal power factors,

and in a factor form, so one surface is left for the impression of the prescriptive focal power required, and the remaining parts vto give the true size eifect independently of the said prescriptive surface. v

Another object of the invention is to provide means whereby size and power lenses or lens systems may be supplied the dispenser in such form that the lens or lens system may be finished by him to required prescriptive power by simply impressing on one surface the said prescriptive power curve, thereby making it possible to dispense these lenses or systems in the same way ordinary ophthalmic lenses are dispensed in the art, instead of requiring the whole lens or lens system to be made to required prescription by a lens factory which would delay the time in providing the desired lenses to the patient and would materially increase their cost to the patient.

Another object of the invention is to provide a method of computation for lenses and lens sys# tems of this character which takes into account all of the factors involved in lenses or lens systems of this nature and separates out the power effect, the true size effect, and the variations for distance from the eye of the lens or lens system and the distance to the object and eliminating later those of vanishing importance, whereby any prescriptive lens of this nature may be expressed by formula and readily designed therefrom, differentiating from prior methods of computation where individual Vprescriptions were each figured independently for their individual condition of use, whereby I am able to codify and systematize the entire range required for use for ordinary and usual prescriptions, instead of having to compute laboriously and expensively each individual lens or lens system requiring a laboratory computation and factory production of each required lens or lens system. Another object of the invention is to provide lens blanks for lenses and lens systems of this character in semi-finished form codified for true magnification or size factor whereby they may be supplied in series of various magnications which be made in the details of construction and arrangement of parts and in the steps of the process Without departing from the spirit of the invention as expressed in the accompanying claims, the preferred forms, steps, and arrangements being shown and described by way of illustration only.

. Referring to the drawing: K

Fig. I is a diagrammatic illustration of a size lens system placed before the eye and viewing an object for the purpose of deriving the generalized formula for lenses and lens systems of the invention;

Fig. II is a cross section of a lens blank of the invention showing a surface left free for impres-l sion of the prescriptive power curve;

Fig. III is a view similar to Fig. II of the same lens blank with a denite prescriptive powercurve impressed on the free surface;

Fig. IV is a view-similar to Fig. II, with a prescriptive curve on the free surface different from that of Fig. III but having the same value of true magnification S1, but a different effective power De;l

Fig. V is a cross section of a two element lens system showing a free surface for the prescriptive power curve;

Fig. VI is a top diagrammatic sectional view of a pair of spectacles or eyeglasses having -lens systems of the invention; l

Fig. VII is a cross section oftwo lenses cemented together for the purpose "of aligning the axes on the two outer surfaces;

Fig. VIII is a cross section of two lenses with an air space between; and

Fig. IX is a cross section of a modied form of the invention showing a separator or a filler piece between the lens elements to `provide for increasing or decreasing the space between said elements without appreciably increasing the weight of the finished lens.

In the prior art, single vision and multifocal y ophthalmic lenses have been dispensed by the factory finishing one side of the lens blank. These blanks were sold to the dispensers who received the patients` prescription and placed on `the unfinished side of the blank a surface .to complete the lens to prescription requirements. -This would be a lens by lens job, would be very expensive for each case, and would cause almost indefinite delay in delivery. The system described above is the universal system of the art in dispensing such lenses and has through practice been reduced to a very efficient and practical i one.

Up to a very short time ago, ophthalmic lenses provided only for focal power, defect in shape of eye, astigmatism, and muscular defects, prismatic displacementI utilizing the spherical curve for power, a cylinder or toric curve for astigmatism, and prism for muscular defect. Recently, however, it has been discovered and Irevealed to theart that there may also be defects in size impressions ofthe two eyes, one eye maSr see larger than theother, or a single eye may have a different size impression in different meridians. 'I'his defect has been compensated for by adding a magnication factor in the lenses or lens systems, one that will change the size impression" or relative size of the ocular images without aliecting the focal power of the lens systems. This factor is introduced by means of the property of lenses to change magnication without change of focal power by means of a change of shape, thickness, distance from the eye, and distance of the object from tbe eye; it is a factor of the shape or form of the lens and not merely its focal power. The prescribing of such lenses is in its infancy. Up tp the present time it has been confined to computing the lens in the laboratory and finishing the lens completely at the factory for each individual prescription, a very costly, laborious, and lengthy proceeding, and one impracticable to the organized methods of making and dispensing ophthalmic lenses. It is a principal object of my invention to avoid these expenses, dela-vs, and laborious proceedings involved in making and.' dispensing lenses of this nature, by providing a simplified and generalized method of computing such .lenses-by a general formula which l'. have derived so the same may be codified and systematized to flllthe usual and general prescriptions in the usual methods of dispensing now in vogue in the art, simplifying and cheapening the computations as well as the methods of production and dispensing by providingthe dispenser with lens blanks as in the present systm which may be converted into nished lenses of required magnification and focal power by simply `impressing a power surface on one surface of the blank left free for that purpose by the manufacturer, and to provide such blanks in series of varied magnications' which may be utilized by the dispenser to meet the prescriptive requirements of individual prescriptions presented to him embodying the correction of size as well as focal power where the combination of the two is required. My invention embodies both new computations and methods of computation as well as a new method of producing and supplyin lenses and blanks of this character.

The majority of size lenses fall within the range from no true magnification to four per cent true magnification.

I assume for the sake of deriving a formula for the imageY size that the eye is stationary. At first, I assume an object at any distance and determine rigorous formulae, then assume distant .through a fixed opening which is the image of the pupil of the eye as formed by the cornea; in otherv Words, assume that there is a fixed opening properly placed like a stop. We can then derive the -following expression for the 4magnification of this lens system:

Mii-UD. c

Where M1=the complete magnification for a disi tant object.

U=the distance from xed opening (entrance window) to ocularsurface of the lens system.

De=the effective power of the lens system.

C=a function of all the surfaces, refractive indices of the glass, thicknesses, and separations, except the power of the ocular surface.

An analysis of this formula shows that the complete magnification for a static eye and dis'- tant object is the product of two independent terms, namely, the first term gives the effect produced by the focal power, and the second gives the effect of the shapes, separations, thicknesses, etc., i. e., the magnification, except that the last ocular surface is left free so a surface may be placed on it to give the desired prescriptive power. I indicate the distant magnification due to power P1, and the magnification due to shape or form S1, then.:

The examination of -the eyes for their errors, o bviously, must determine their refractive corrections which is De and which, because of unavoidable thickness of the test lenses does contain some C or S1 but which can lbe allowed for, and likewise must contain the complete power magnification P1.

Next let us consider U so wecan determine P1. It is measured from a point roughly four millimeters on the retinal side of the cornea to the ocular surface of the test lens, but we need not know its accurate value, in fact, ,if we put the ocular surface of the prescription lens at the same place as the ocular surface ofthe test lens wedo not need its value at all, since P1 for the precriptive lens'will be the same as'P1 for the test lens system. Roughly, U is twenty Vmillimeters, since everything in lens Ltheory is expressed in meters, U=0.020 meters. l

If the test lens is placed at a dierent position than the prescription lens there is'a change' in P1 due to change in U, likewise De must b e changed or corrected, as is well known. This leaves S1 or the true magnification uncontaminated with varying degrees of corrected eye focus.. The commercial importance o f 1 Slw v i is that C does not contain the ocular surface,

and in consequence any necessary ocular` sur face can be ground into the system to give the required value of De or the focal power correction. In other words, semi-finished blanks can kbe tabulated and stocked giving magnification Sl and a curve placed on the ocular s ide to give the prescription desired, This eliminates a great deal of diiculty in transcription and stocking of lenses of this nature.

We next consider the effect of position of the prescription in front of the eye, assuming the eye fixed; a change in the position of the prescription in front of the eye is a change in U. For the effect of a change in U, we can make an approximation to our exact equation:

P1=l+UDe approximately, or in percentage and millimeters;

(P1-1) in percentage =1/4.%, times the change in U in millimeters for a value of De of 2.50 dioptres (an average prescription).

Since we must keep the product P S constant we must change S if we change P.

If De is more, the change is more and vice versa.

'I'he determination of the position of the test lens and the prescription lens must be accurate. The most accurate measurement that can be physically made is from the front surface of the prescription lens to the front surface of the cornea, but U should be measured from the ocular surface, i. e., the ocular surface of the test lens should be positioned the same with respect to the cornea or the ocular surface of the prescription lens as is common ophthalmic practice.

It will be seen from the formula 1= M 1- UDe X c that in the part of the expression I have collected all of the elements involving the focal power of the lens system, expressed as De the effective power of the lens as it is ordinarily measured combined with a distance U which indicates the position the lens is placed before the eye, while in the portion I have collected all the elements which are inde` and which is called herein S1.

It is because of this separation of the equation into the two groups that I am able to provide lenses on which different powersA of ocular surfaces may be imposed without affecting the true magv nification of the lens, thereby making it possible to dispense the lenses in the usual waythat ophthalmic lenses are dispensed by the dispensers in that art, which has hitherto been considered impossible.

The free or excluded surface which I have is ordinarily the ocular surface of the lens system, i. e., the surface nearest the eye.

My method of analysis is based on the principles of Gauss as laid down by Pendlebury in 1884 with my necessary extensions to that theory.A

'Ihe following considerations and symbols are used in this analysis:

The direction of incident light, left to right is 5 positive. All rays measured in this direction are positive.

A radius of curvature, convex to the incident light is positive.v

The order of indices of the refractive lmedium l0 are indicated, pm, p1, ,11.2, etc. m

Surface powers,

etc. l5

Thickness -r positive; reduced thickness etc., are negative, but when s is used for a reduced thickness it is positive; likewise, when D (diopter) is used in place of p it has' the conventional Value y as ordinarily used in ophthalmic practice.

Referring to Fig. I the following is an explanation of the symbols usedz- Object l is imaged into lm. Distance object to lens system=d=u Distance image to last surface of lens=v Angular size of object from stop point wo Angular size of image from stop point m Magnification (angular) tan :en tan w Distance stop from lens system U. Total thickness of lens system 2T Linear magnification L- m I t ml d+2T-1-U an wn*UAI :d4-21+ U ',--(U-v) but from Pendlebury y 1 U(B-Ad)+(cd-D) M* d+2f+ U A, B, C, and D are expressions from Pendlebury and are given later.. l. B-ut if the object is 'at avdistance, d is large in comparison with everything else. Call the magnication for a distant object M1 instead ofy M, 60 we have 1 mr-AUJFC from Pendlebury 1 A `-1 w t-e-De p f and C does not contain the last surface.

:.1%= U.c.De+C=,c(1-UDe) l 70 Note that De also the D which refers to'surface powers is not the f 52A ama/m Then if we call the part of the magnication for a distant objectvdue to the power P and that due to shape S1 then Pl 1 UDe De is what vis commonly called the effective power'or vertex refraction of the lens andis actually the reciprocal of the back focal length of y the -lens expressed in meters.

The form of A, B, C, D is obtained as follows n ie la A B m Capn smpn also ac gg: :571;:0 3px o is an expression'indicating the partial derivative of A with respect to p1; etc.

' A1=p1 l A2 -s Azn1=Ann-a0mni+IHPHm by means f which Athe equationforfn surfaces can be obtained.

Fo'r two surfaces Aa--m'l'pa-l-Pxpz =1+p1 or in terms of D and s opthalmic notation Aa= D1 i- D2'fSiD1D2l C=. 1 SDIS papapitata plpzpapititzta -lpxpspdita pip4t2 paps -I- plpnpdifz-l-pipiti The important Iterms for a distant object are collected below (n is an air space between lenses) I UB-AUd-l-cd-D M 10i-Liv) but A=CD and approximately for four surfaces; since n=.-,ui `t1 etc.

` approximately approximately Therefore This formula includes second order terms but not third order, and gives the differences between the reciprocals of the magnification of a near and a distant object. y

A semiiinished lens with all surfaces nished except the prescription surface p4 will have all the quantities except the p4 already compensated for in the design. This leaves only UX U (p1+pz+ pa+p4) which is already eliminated when U is the same with the test lenses as with the prescription lenses and (pi+pz+ps+p4) is the approximate value of the actual power prescription, and the terms Updtr-i-tz-i-ia) |Up42f which is equal t0 which cannot be completely compensated for in the semiiinished blanks since pi will vary with the power of its prescription. However let us limit the range oi powers over which a given semiiinished series of blanks is to be used,- then we will know the approximate value of this term. For an extreme range we can take p4 to vary from 5D to 15D which is 5D each side of the mean, then we can take =1.5, the total thickness 2r as 0.010. and U=0.02\ then the error in 0.02X5X0.0l0X0.3

or if this is reduced to per cent in magnification I we have 0.98\%^which is closer than required and can be further reduced i! desired.

It is thus seen that seminished blanks can be made practically so that the prescription curve can be placed on one surface4 for near as well as for distance.

For the discussion of Pendlebury referred to above see Lenses and Systems of Lenses Treated after the Manner of Gauss by Charles Pendlebury, M. A., FRAS, published, Cambridge, England 1884 The lens shown in Fig. II comprises a lens element having the surface 1 and a thickness greater th-an -r, say -r-l-as, so that the lens maybe finished to the thickness r. 'Ihe surface 1 is a nished optical surface. The surface 2 may be left unfinished for a purpose to be described later.

To start with, say we desire a lens having a certain S1, or true size magnification. Then we have 1 1= s 1-SD1 where D1 is the surface power of I and s is the tl.ickness -r divided by the refractive index of the. glas's. It will be seen from this formula that eit. ier small or large values of s can be used provid ed we use with the smallvalues, large values of Di and vice versa, so We choose a good average value of both s and D1 for a commercial lens, which values are so chosen as to satisfy the above equation.

Then we compute the eiective power` or vertex refraction of the lens, assuming the surface 2 to b at or plano.

Then if we wishA a lens with no focal power,

. we put on surface 2 the focal power computed with opposite sign, as for example, if say, a flat surface 2 gives an effective power of plus 6 diopters, we would for a zero power lens grind a minus 6 diopter surface curve on the face 2.

If we wished a power of plus 1 diopter, wewould grind on the face 2 a minus 5 diopter curve and so on. Whatever the surface ground on the face 2, the thickness -r must be preserved yfor vthe nished lens.

When the eye has been tested, the magnication due to the power of the lens has been placed in front of the eye in the test lenses, so we do not need to include P1 of the formula, unless we wish to change the distance the prescription lens is to be placed before 'the eye when it is to be other than that of the trial lens. When we do make this change of distance this obviously changes U in the formula for P1 and must be allowed for. Y

In the test lenses, if they were very thin, they l ,would involve .no shape magniiication, but actually they are not very thin, so there is some S1 In Fig. III there is shown the same lens as Figf I, except that a power curve has been placedl on the face 2 to show a lens of zero power.

In Fig. IV there is shown a lens the same as Fig. I, but a diierent curve has been placed on the face 2 to give a different focal power to the lens, but all the lenses of Figs. III and IV have the same true size magnification S1.

i In Fig. V there is shown a lens system of tWo separate lens elements 3 and '4, having surfaces 5, 6, l, and 8, and thicknesses r1,- rz, and r3, where -rz is an air space. The surface 8 has been left unfinished, and the actual value of fs is n+1', as explained before, the :c to be ground away whenl 4.6 millimeters.

. nication P1 equal to surface 8 is finished to required prescription curve. In computing this lens we use the extended formula for a sequence of four lens surfaces instead of the two of the lens of Fig. I. 'Ihis formula for S1 for the four surfaces does not contain the fourth surface butonly its position. 'Ihis lens system, as 'for the lens system of Fig. I, gives a lens system of required true size magnification S1 and a free surface 8 to be varied' as required to give required focal prescription power of the lens system.

Applying the formula IWT-*1 UDe X--PXS1 to the lenses in Fig.

We have no De in this lens because it is a blank with surface I finished according to the C formula and/the thickness of the ultimatelens determined by the same formula.

' In Fig. III we have a lens with De equal to zero and the lens is finished to the thickness r as described for Fig. II with a curve on the ocular side 2 such that there is zero De power.

In Fig. IV we have the same lens finished to give a De of plus one diopter focal power.

To determine the curve I and the thickness 1- of all these lenses, i. e., Figs. II,1III, and IV, We assumed that we required a magnification 1.8 per cent, which makes S1=1.018- Which is 1.8 per cent greater than unity. Then we have for the true size magnification (S1) equals one divided by l-siDi of the formula. Thus, if we take D1, the power of surface 1 equal to plus six diopters s is 0.003, but s is the so called reduced thickness,

therefore it must be multiplied by the index of refraction of the glass to get the actual glass thickness r of the finished lens, which is 0.003 times 1.52 equal to 0.0046 meters or as is com. monly expressed 4.6 millimeters. Thus we have of 4.6 millimeters. We have not carried out this example to the number of decimal places that we would in actual lens design.

If for other reasons we wish to make the surface I steeper or less steep, we can change r to correspond and get the same magnilcation, so long as we' use the formula In Fig. III we have De=zero, so that we make surface 2 slightly stronger than surface I to make the effective power De=zero by the regular formula. This means that this surface has a power of 6.12 diopters to the nearestM; diopter tool available. This lensKhas a true size magnification 1.8 per cent with no power and no power magnification. f

In Fig.v IV we have put on the ocular surface of this lens a surface power of 5.12 diopters and the thickness as previously determined of This lens has a true size magnication of 1.8 per cent and also a power mag- 1UDe However, it is not necessary to compute P1 since this part of the magnification due to De is already in the test lenses. Also, because of the nite dimensions of the test lenses there is -a a front surface I of 6 diopters and a thickness f small shape magnification due to their thickness andV shape in addition tothe size correction froml the size lenses, but in uniting the prescription the smallshape correction of the-test lenses is combined with the size correction found from the size lenses.

In the lens system of Fig. V we use the formula M1:P1XS1 where S1 is as required, and get the same true magnification,

S1, from the formula of C'z. After the surfaces and thickness have been determined to give the magnication S1, we can put the ocular surface on this lensto give the required value 'of De by the usual formula for effective power.

'I'he lens system of Fig. V has the following characteristics:

Indexl of glass 1.5.

Radius of surface 5:50 millimeters, giving a surface power of plus 10 diopters. v

Radius of surface 6:60 millimeters, or surface power of minus 8,33 diopters.

Radius of surface 7:70 millimeters, or surface power of plus 7.14 diopters.

'Ihickness r1=2 millimeters, giving t1 of minus 0.0 013 meters. 'Thickness l11:0.6 millimeters, giving reduced t: since this is an air space=minus 0.0006 meters.

'I'hickness r3-:3.5 millimeters, or a reduced thickness t3 minus Y0.0023 meters.

This surface 8 is to be determined by the power Ds desired in the prescription.

the reduced Index v1.5

1.5-1 #5 50mm. radius p1= 05 =+10D i 1. 1.5 #6 60 mm. radius I,0g- 0 060 8.33

1.5-.1 #7 70 mm..radius pa--o-T +7.14

p1f1= 0.0130 pzh: t0-.0050 pata: '-0.0164

plpgtlta-F '-0.0002 plpatzyta: i 0.0001 I pipzpafitzts: 0.0000

, p'lpamF-l-coooz maar.: 0.0001

pgpgtltg: '-0.0001 Therefore C1:1 -0.0343:0.9657 and S1:1.0355 or the valueof the true size magniiicationvfor this lens is 3.5%. The required power De, can be computed for any value of p4 or the value Of The lens was figured as follows from formula p4 can be determined for any value of Der quired by well known computations. In4 Fig, VI there is shown a pair of lenses 9 and I0 mounted in a frame before the eyes. Let

us assume that the eye in front of which is mounted the lens 9 requires a given amount of true magnification S1 over that of the other eye. There will be some lunavoidable magnification'in the lens I0, so we must make S1 of the lens 9 larger than S11 by this amount, as for example, suppose the eye which lens 9 is in front of requires a true magnification of 1.02 and the lens I0 has a true shape magnification of 1.01, then we must make the lens 9 to have a size magnication S1 equal to the product of 1.01 multiplied by 1.02`J which gives about 1.03 for-the shape magnification required; in other words, the ratios of the magnications of the two lenses must be the requiredY amount to give thef right size correction to the eyes.

In Fig. VII there is shown a two .element lens system composing the elements Il and I2 tted together on their contacting faces I3 and secured together by cement or'otherwise to make a unitary lens structure. i

In Fig. VIII there is shown a. two element lens system, comprising the elements I4 and I5 with an air space I6 between them. 'I'he two elements are fitted and secured together adjacent their marginal edges to form a unitary lens structure.

The structures of Figs. VII and VIII are particularly important where the true size magniiication is Adifferent inA one meridian than inf the other, and in consequence requires a toric surface on a face .of each part because the 'torio axes may be'easily aligned after they are finished by rotating one element on the other, it being a very dicult and expensive operation to align toric axes in. on piece structures with suiiicient accuracy. f f

In Fig. IX there is shown a modified form of the invention wherein the lens elements I'I and I8 are held in spaced relation by a spacer member or' filler piece I9 of glass or other suitable means which is'varied in thickness to increase or decrease the space between the lens elements and thereby increase or decrease the magnification without appreciably increasing the weight of the finished lens. The edges of the lenselements I1 and I8 may be faced as shown at 20 to receive the filler piece or the said filler piece may be shaped as shown at 2l to receive the lens elements.

In all of the above figures, the letters OC indicate the surface on the eye side of the lens on which the nal prescriptive curve is to be formed to .finish the lens.

The lens blanks of this invention may be supplied as single units, forfvarious values of" S1 either spherical or toric, in the latter case there are two values of'S1 for each blank. lA desired prescription may be lled by the dispenser by picking out a blank with the desired S1 value and placing'on the free face the required prescription curve to give the desired focal power.

Theblanks may be also supplied in series of different magnications graded. to meet usual practical requirements.

The surfaces may be spherical, cylindrical, torio, prismatic, aspheric or any of the surfaces of prior art lenses and ground and finished in the usual prior art wayvby prior art methods and for the general purposes of prior art corrections.

The lenses may be given any desired outline shape and will adapt themselves to practically The definiteV reduction from a distant object to a near object is shown by the formula set forth and can be applied where necessary but for practically all the ordinary cases the reduction is so small as to be neglectable since it is less than the tolerance of the eyes.

No specific mention has been made of bifocal lenses but they fall directly under the formulas given herein, except that there are three surfaces often instead of four surfaces. The three surface formula is derived from the basic differential equations or may be derived from the four surface equations by putting the first thickness equal to Zero and the rst power equal to zero, and in case of a fused bifocal, substituting the correct values of the indices of refraction that are actually used in the lenses.

The expressions, true magnification, o-r shape magnification, etc. are used for the magnification due to the shape and thicknesses, etc., as distinguished from the power magnification that would be produced by an infinitely thin lens having the same focal power as the lens combination actually used and placed at the same distance from the cornea as the ocular surface of the lens combination.

An itemized list of the factors in the lens of thef invention-will make the invention clear. This statement of the elements of the lens and their functions and relationships,`it is believed, will make the invention clear at a glance.

The elements are:

l. The index of refraction of the lens medium or glass.

The lens units are made-of the ordinary optical crown glass usual in the art for making ophthalmic lenses. The lenses may be one part or multiple part lenses. While the index of refraction f separate parts may be different, all the lens partsof multiple part lenses are generally made of the` same index of refraction. This lens involves no new elements of the index of refraction; hence, it may be eliminated as an important inventive factor. The same rules and requirements of the index of refraction existing in ordinary prior art ophthalmic lenses apply and exist equally in the case of this lens.

2. The number of parts of the lens.

The lens may be single or multiple part, as explained above. 3. The distance from the eye of the lens sys- The distance from the eye should be the same as the corresponding distance of a particular part of the lenses with which the eyes are tested. If to be used at another distance, compensation will have to be made. The distance from the eye in general affects the magnification due to power, but not the shape magnification, except tc a negligible extent; hence, as far as shape magnification is-concerned the distance from the eye can far distances, as understood in the art. Magni-i fication due to power introduced by the ocular surface will be affected slightly by alteration of .this distance.

5. The surface curvatures. The shape magnification in my lenses is conl trolled by the surface curvatures of all of the surfaces of the lens system, except the ocular surface. 'Ihese surfaces may be spherical for overall corrections, or cylindrical or toric for meridio-nal corrections. They may be any of the usual lens surfaces generated by the usual prior art methods. The front surfaces affect the shape magnification; all of the surfaces, including the ocular surface, affect the focal power; hence, a shape magnification blank may be made by disregarding the ocular surface, by making the vsummation o-f the surfaces for a requisite thickness to give a focal power of zero, and a required shape magnification; then the ocular surface may be changed to give a required focal power without altering the shape magnification, thus allowing the use of many different ocular surfaces all with the same shape'magnification, and thus producing a new magnification lens. The ocular surfacegwill introduce a magnification due to power, but as this magnification is the same as that existent in the trial lenses by which the eyes were tested, it is equalized and may be disregarded.

6. The thickness and separation of the lens parts.

The central thickness of a single part lens, and

the thickness plus the separations of multiple part lenses is a factor in the shape magnification, and cannot be changed without changing said magnification. The thickness, hence, must be held and the ocular surface put on to that thickness so the" shape magnification will not be changed. Introduction of the ocular surface will introduce magnification due to power of the same amount as the trial lenses.

These are all ofthe elements of the lens of the invention. The process of making the lens may be briefly described as making a shape magnification lens of desired shape magnification with an ocular surface to give zero focal power and adding, the focal power required to this ocular surface to give a required focal power without changing the shape magnification, magnification due to power beingthe same as that of the'trial lenses. In this way one blank of a given shape magnification may be used for many lenses having different focal powers, thus adapting the lens to the method of distributing prescription lenses in vogue in the art.

The term spectacle as used herein implies and includes any and all means for mounting and holding lenses or lens systems before the eyes such as spectacles, eyeglasses, goggles or any other form of mounting.

From the foregoing it will be seen that I have provided 'a new computation of lens systems of this'character and have provided new lenses to give the desired corrective results by which the computation, manufacture, and dispensation of lenses of this character are materially simplified and cheapened, and by which service to the public'is materially facilitated. Y

Having described myinvention, I claim:

1. 'A spectacle. lens for use in combination withanother spectacle lens system for the other eye, for equalizing the measured size difference of images of the two eyes, having prescriptive shape magnifications and prescriptive focal powers for a given distance of object and a given position dium and D1 is the front surface power of said um and D1 is the front surface power of said meridian-and a rear or ocular optical surface of zero effective shape magnification power, which combined with the said front surface, thickness and index of refraction and the position of said ocular surface with relation to the eye, will produce the prescriptive focal powers without change of the said shape magnifications.

2. A spectacle lens for use in combination with another spectacle lens system for the other eye, for equalizing the measured size difference of images of the two eyes, having` prescriptive shape magnifications and prescriptive focalpowers for a distant object and a given position before the eye in the two major meridians of the prescription, both for shape magnication and focal power, comprising Aa pieceof lens medium of given refractive index, a front optical surface and a thickness which combined together for a lens medium of said refractive index will produce the prescriptive amount of shape magnifications, in which -wherein'S1 is the required shape magnification,

for each of the major meridians,` s1 is the thickness divided by the refractive index of the memeridianY and a`rear or ocular optical surface of zero effective shape magnification power, which combined with the said front surface, thickness and index of refraction and the position of said ocular surface with relation to the eye, will prol duce the prescriptive focal powers without change of the said shape magnifications.

3. A lens blank for a spectacle lens for use in combination with another spectacle lens system for the other eye for equalizing the measured size difference of images of the two eyes, having pre- .scriptive shape magnifications and prescriptive .focal powers for a given distance from the eye and a given distance of object in the two major meridians of theprescription, both for shape magnification and .focal power comprising a piece of lens medium of given index of refraction, a

front optical surface and a thickness which combined together for a lens medium of said index of refraction will produce the prescriptive shape magnifications in which wherein Sl isthe required shape magnification, f or each ofthe major meridians, s1 is the thickness divided bythe refractive index ofi the me-A l dium and Di is the front surface power of said` meridians, and an excess of material in the direction of the thickness on the ocular side to provide for the placing on said ocular side of an optical surface of zero effective shape magnficalscriptive amount of shape tion power, which combined with saidfront sur`\ face and thickness and index of refraction lfor producing the said shape magnificationsat the required distance of object and position before f the eye will produce the prescriptive focal powers without'changing said shape magnifications.

4. A lens blank for a spectacle lens for use in combination with another spectacle lens system for the other eye for equalizing the-measured size difference of images of the two eyes, having prescriptive shape magnifications and prescriptive focal powers for a given distance from the eye Vand for a distant object in the two major meridians of the prescription, both for shape magnification and focal power, comprising a piece of lens medium of given index of refraction, a front optical surface and a thickness which combined together for a lens medium of said index of refraction will produce theJ prescriptive shape magnifications in which wherein S1 is the required shape magnification, for each of the major meridians, s1 is the thickness divided by the refractive indem of the medium and D1 is the front surface power of said meridians, and an excess of material in the direction of the thickness on the ocular side to provide for the placing on said ocular side of an optical surface of zero effective shape magnification power, which combined with said front surface and thickness and index refraction for producing the said shape magnifications at the required distance of object and position before the eye will' produce the prescriptive focal powers without changing said shape magnifications.

5. A spectacle lens system for use in combination with another spectacle lens system .for the other eye, for equalizing the measured size difference of images of the two eyes, having prescriptive shape -magnifications and. prescriptive focal powers for a given distance of object and a -given position before the eye in the two major meridians of the prescription, both for shape magnifications and focal powers, comprising'a plurality of pieces of lens mediums of given indices of refraction, optical lens surfaces, on al1 of the faces of said pieces-of lens medium except theA ocular side of the piece nearest the eye, thicknesses of said pieces. of lens medium, separations N of said pieces and indices of refraction thereof which combined together will produce the premagnifications in Which wherein S1 is the requiredshape magnification,

for each of the major meridians and C is a mathematical combination of all the thicknesses, separations, indices of refraction and surface powers', except the ocular surface of said pieces of lens medium and a rear or ocular optical surface of zero effective shape magnification power whichcombined with the said other optical surfaces,

thicknesses, separations,I indices of refraction and thel position of said ocular surface With relation to the eye will produce the prescriptive focal powers withoutychange `of the said tions.

6. A spectacle lens system for use in combination with another spectacle lens system for the other eye, for equalizing the measured size difference of images of th'e two eyes, having preshape magnifica.-

V scriptive shape magnications-and prescriptive focal powers for a distant' object and a given position before the eye in the vtwo major meridians of the prescription, both for shape magniiications and focal powers, comprising a plurality of pieces of lens medium of given indices of refraction, op-

tical lens surfaces on all of the faces of saidv wherein S1 is the required shape magnificationV for each of the major meridians and C is a mathematical combination of all the thicknesses, separations, indices of refraction and surface powers,

except the ocular surface of said pieces of lens medium and a rear or ocular optical surface of zero effective shape magnification power which combined with the said other optical surfaces, thicknesses, separations, indices of refraction and the position of said ocular surface with relation to the eye will produce the prescriptive focal powers without change of the said shape magnifications. I

7. A spectacle comprising a pair of' lens systems, one for each eye, said lens systems having a prescriptive ratio, one to the other, of shape magnifications, and each having prescriptive focal powers for a given distance of object and a given distance from the eyes, in both the major meridians of the prescriptionsor each eye, comprising for each eye lens mediums of given indices of refraction, thicknesse; and separations and optical lens surfaces on al of the faces of said mediums, except the ocular side of the me- 40 diums nearest the eyes, which combined together tween the two eyes, in which wherein S1 is the required shape magnification for each of the major meridians of each eye and C is a mathematical combination for each eye of all the thicknesses, separations, indices of refraction and surface powers, except the ocular surfaces of said mediums and a rear or ocular optical surface of zero effective shape magnification power for each eye which combined with the said other optical surfaces, thicknesses, `separations, indices of refraction and position of said ocular surfaces with relation to the eyes for each eye will produce the prescriptive optical powers for each eye Without change of said shape magnications.

8. A spectacle comprising a pair of lens systems, one for each eye, said lens systems having a prescriptive ratio, one to the other, of shape magnifications, and each having prescriptive f0, cal powers for a distantobject and a given distance from the eyes, in both the major meridians of the prescriptions for each eye, comprising for each eye, lens mediums of given indices of rerefraction, thicknesses and separations and optical lens surfaces on al1 of the faces of said mediums, except the ocular side of the mediums nearest the eyes, which combined together for said mediums for each eye will produce the prescriptive ratio of shape magnications between the two eyes, in which wherein S1 is the required shape magnification for each'of the major meridians of each eye and C is a mathematical combination for each eye of all the thicknesses, separations, indices of refracocular surfaces With relation to the eyes for each eye will produce the prescriptive optical powers for each eye without changeof said shape magl nications.

EDGAR D. mLYER. 

